Stability of 5,5-dimethyl-1-pyrroline-N-oxide as a spin-trap for quantification of hydroxyl radicals in processes based on Fenton reaction
Graphical abstract
Introduction
Over the last two decades, interest in Advanced Oxidation Processes (AOPs) and Electrochemical Advanced Oxidation Processes (EAOPs) for the degradation of various organic pollutants has been growing constantly (Martínez-Huitle and Brillas, 2009, Oturan and Aaron, 2014). Although AOPs and EAOPs include different types of processes such as chemical, photochemical, sonochemical and electrochemical processes, they all aim to produce hydroxyl radicals (OH). Indeed, OH is a powerful oxidative agent (2.80 V/SHE) and a highly reactive species able to attack organic pollutants with second order rate constants ranging from 107 to 1010 L mol−1 s−1 (Lhomme et al., 2008). No matter which AOP, the generation of hydroxyl radicals is a key parameter to control and optimize in order to improve the efficiency of the process and to elucidate the degradation mechanisms. However, the very short lifetime of OH (about 10−9 s) makes its detection and quantification difficult, thus leading to the development of indirect detection methods including UV–Vis spectrophotometry (Zhao et al., 2015), luminescence (Tsai et al., 2001), fluorescence (Xiang et al., 2011) and other electrochemical and HPLC methods (Si et al., 2014). However, all of these analytical methods measure neutral products after hydroxyl radical was scavenged. The hydroxyl radical concentration is then inferred from the product concentration instead of measuring radical concentration directly. Although electron paramagnetic resonance (EPR) could not directly measure short-lived species such as OH, this technique could detect longer-lived radical adducts generated by using spin-trap agents at concentration as low as 10−14 M (Haywood, 2013). To date, the very high majority of spin-trap agents used for the detection of OH are nitrone compounds, mainly because they lead to the formation of relatively stable nitroxide radicals that can be detected by EPR. The current challenges mainly concern the improvement of the reactivity and stability of the spin-traps agents for a more reliable detection. Among them, α-phenyl-N-tert-butylnitrone (PBN), 1,1,3-trimethylisoindole N-oxide (TMINO), 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO) or 5-methyl-1-pyrroline N-oxide (DIPPMPO) have given interesting results, especially under biological conditions (Bottle et al., 2003, Chalier et al., 2014, Luo et al., 2009, Timmins et al., 1999). Nevertheless, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) is still the most commonly used compound. Its reaction with the previously cited species leads to the formation of relatively stable aminoxyl radicals detectable by EPR (Makino et al., 1990). Since the late 80s, the behavior of DMPO for the detection of hydroxyl radicals has been extensively studied, mostly in biological systems. Indeed, the detection of free radicals in living bodies has been of a great interest ever since they were suspected for being responsible of various diseases such as hypertension or cancer. It has been assumed that Fenton reaction is the most important pathway for hydroxyl radical generation in tissues (Yamazaki and Piette, 1990). Most of these studies were carried out with concentrations in hydrogen peroxide and ferrous iron hardly exceeding dozens of μM. To our knowledge, studies under conditions similar to those used in AOPs and EAOPs systems based on Fenton reaction (chemical Fenton, photo-Fenton, electro-Fenton, photoelectron-Fenton, etc.) are rarely reported, despite being of a high interest for these processes. In these systems, concentrations in Fenton reagent are much higher than those reported in biological systems. Indeed, concentrations in H2O2 and Fe2+ generally could range from hundreds of μM to hundreds of mM, and H2O2 concentration can even reach dozens of moles per liter for treatment of wastewaters containing high chemical oxygen demand (COD) loads (Brillas et al., 2000, Gulkaya et al., 2006, Martínez-Huitle and Brillas, 2009). As a result, the stability of DMPO and its OH spin adduct (DMPO-OH) in presence of high concentrations of iron and hydrogen peroxide become a critical issue in accurate quantification of hydroxyl radical concentration, because of the formation of by-products (paramagnetic or not) affecting the reliability of DMPO-OH (and thus OH) detection (Buettner, 1993, Li et al., 2007, Makino et al., 1990).
Fenton reaction has some well-known drawbacks such as large amount of sludge formation due to the precipitation of ferric hydroxides, scavenging of hydroxyl radicals by hydrogen peroxide, etc. In fact, AOPs and EAOPs are currently being developed to overcome these limitations, showing excellent potential for the treatment of wastewaters containing dyes (Brillas and Martínez-Huitle, 2015), pharmaceuticals (Antonin et al., 2015), pesticides (Zazou et al., 2015), and also for the treatment of municipal sludge (Fontmorin and Sillanpää, 2015). In order to improve their development in particular by gaining more fundamental understanding about the generation of OH in these systems and about the degradation mechanisms involved, EPR is the most direct analytical instrument. In this study, we developed a theoretical model to determine under which conditions DMPO should be used to act as a reliable spin-trap for the quantification of OH generated in AOPs based on Fenton reaction. To our knowledge, this is the first study that quantitatively shows the impact of DMPO concentration, relatively to Fenton's reagent concentration (i.e. H2O2 and Fe2+), on the stability of the adduct in AOPs conditions. Therefore, this work could be useful to further understand the behavior of DMPO and other spin-trap agents in such systems, but also to optimize AOPs with a more reliable quantification of hydroxyl radicals.
Section snippets
Chemicals
Hydrogen peroxide (H2O2, 30%) and iron sulfate (FeSO4·7H2O) were purchased from Merck (Damstadt, Germany). Sodium sulfite (Na2SO3) and catalase from bovine liver were acquired by Sigma-Aldrich and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was purchased from Caymen Chemical Company (Ann Arbor, USA). All chemicals were of analytical grade and used without further purification.
Analysis: EPR and UV–Vis measurements
EPR data was obtained using an electron spin (paramagnetic) resonance spectrometer CMS-8400 from ADANI, Minsk, Belarus. The
Impact of Fenton reagent concentration on EPR background signals
The impact of Fenton reagent concentration was tested with [DMPO]: [H2O2] ratio varying from 1 to 100. A typical spectrum of DMPO-OH is presented in Fig. 1a: it is composed by a characteristic 1:2:2:1 quartet with hyperfine couplings aN = aβH = 1.50 mT (computer simulation, Fig. 1b). The amplitude of the DMPO-OH signal was followed during the time for different concentrations of Fenton's reagent, and results are presented in Fig. 2. As depicted, the concentration of Fenton reagent had a
Conclusions
In this study, the impact of DMPO concentration for the detection of hydroxyl radicals under conditions similar to AOPs/EAOPs based on Fenton reaction has been investigated. Impact of high concentrations of Fenton reagents (up to 100 mM H2O2 and 10 mM Fe2+ with a constant ratio of 10: 1) was studied.
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Experiments were based on a theoretical model developed to determine the optimal DMPO concentration for a reliable detection of hydroxyl radicals.
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In our system, for a reliable detection of hydroxyl
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